The present invention relates to mountings for bearings which are to transmit high loads, particularly moment loads from an upper structure to a base structure and such a mounting in which the bearing is to be relatively easily disconnected from one of the structures. The invention is particularly useful in material handling apparatus, such as cranes and earth handling machinery having a rotatable upper structure which is releasably connected to a base or carrier by a bearing.
Cranes and other material handling apparatus commonly have what is referred to as an upper structure which is rotatable to horizontally swing a boom thereon. The upper structure commonly includes an operator cab and controls and machinery for operating the boom. A bearing is provided for supporting the upper structure on the base or carrier to provide for rotation of the upper structure about a vertical axis and for transmission of axial, radial and moment forces from the upper structure to the base.
In cranes and other material handling machinery, it is highly desirable to be able to quickly connect and disconnect the upper structure with respect to the base. The ability to do this enables the bearing to be readily repaired, maintained or replaced. It also enables the upper structure to be disconnected from the base so that the base and upper structure can be transported separately to a new job site. This frequently occurs when mobile cranes are involved. Depending upon the size of the crane or other material handling machinery with a rotatable upper structure, highway axle load limits may be exceeded and difficulty may be encountered in clearing low bridges. In the case of mobile equipment the upper structure from the base may be shipped on a flat bed truck while the stripped mobile carrier moves by its own power.
Commonly, bearings on cranes and the like for releasably and rotatably mounting the upper structure on the base or carrier involve bolts which extend through the base or carrier and a bearing member. Nuts are threaded onto the bolts and are torqued to provide high clamping forces between the bearing and base and an axial preloading for the bolts. A connection of this type is shown in U.S. Pat. No. 3,125,226 to Mork et al. To transmit high loads, and particularly high moment loads, through the bearing between the rotatable and stationary structure, these bolts must be preloaded to a uniformly high value. This requires considerable labor and a substantial amount of time in view of the large number of clamping bolts. It is also difficult to release the nuts when it is desired to separate the bearing from one of its mounting structures. Moreover, when a varying axial load is applied to the bolts as a result of a working load applied to the rotatable structure, they are susceptible to fatigue failure, particularly as bolt preload is lost through embedment or lack of maintenance.
In a known interference bolt which has been considered for use with crane bearings, the bolt has an internal hydraulic chamber into which oil may be supplied under high pressure to elongate the bolt. When assembling one member to another with such a bolt, high pressure is established in the chamber to elongate the bolt so that it may be readily inserted into bolt holes in the member. A nut is threaded onto the bolt to a clamping position, and then the oil pressure released so that the elongated bolt resumes its original shape to provide axial preloading and a radial interference fit with the bolt hole. Such bolts, however, weaken the inherent clamping capacity of the bolt because of the removal of material along the axis of the bolt to form the hydraulic chambers. Additionally, high hydraulic pressures are required to achieve significant preloading elongations, and the bolts are non-standard, high cost devices requiring close tolerance machining to be effective. Moreover, the principle involved inherently limits its use to large bolt or stud diameters. Obtaining uniformity of preloading of a plurality of bolts is also a problem since the preload is sensitive to the position and/or clamp load of the nut on its initial setting.
Hydraulic bolt tensioners are also known for their applicability to the tensioning bolts. These are tools which connect to the end of the bolt and tension the latter after which the nut is seated to maintain the preload after the tool is removed.